docs: add missing toPromise to README (#122)

README.md

@@ -113,6 +113,7 @@ The package includes the following rules (none of which are enabled by default):
 | `rxjs-no-subject-unsubscribe` | Disallows calling the `unsubscribe` method of a `Subject` instance. For an explanation of why this can be a problem, see [this](https://stackoverflow.com/a/45112125/6680611) Stack Overflow answer. | None |
 | `rxjs-no-subject-value` | Disallows accessing the `value` property of a `BehaviorSubject` instance. | None |
 | `rxjs-no-tap` | An alias for `rxjs-no-do`. | None |
+| `rxjs-no-topromise` | Forbids the use of the `toPromise` method. | None |
 | `rxjs-no-unbound-methods` | Disallows the passing of [unbound methods](https://ncjamieson.com/avoiding-switchmap-related-bugs/) as callbacks. | None |
 | `rxjs-no-unsafe-catch` | Disallows unsafe `catch` and `catchError` usage in [NgRx](https://github.com/ngrx/platform) effects and [`redux-observable`](https://github.com/redux-observable/redux-observable) epics. | [See below](#rxjs-no-unsafe-catch) |
 | `rxjs-no-unsafe-first` | Disallows unsafe `first` and `take` usage in [NgRx](https://github.com/ngrx/platform) effects and [`redux-observable`](https://github.com/redux-observable/redux-observable) epics. | None |